diff --git a/src/main/java/de/neemann/digital/core/ObservableValue.java b/src/main/java/de/neemann/digital/core/ObservableValue.java
index e42f4120b..f6f2bbf52 100644
--- a/src/main/java/de/neemann/digital/core/ObservableValue.java
+++ b/src/main/java/de/neemann/digital/core/ObservableValue.java
@@ -46,9 +46,12 @@ public class ObservableValue extends Observable implements PinDescription {
 
     /**
      * Makes this value a constant value
+     *
+     * @return this for chained calls
      */
-    public void setConstant() {
+    public ObservableValue setConstant() {
         isConstant = true;
+        return this;
     }
 
     /**
diff --git a/src/main/java/de/neemann/digital/draw/model/Net.java b/src/main/java/de/neemann/digital/draw/model/Net.java
index f838a534d..d4a75206e 100644
--- a/src/main/java/de/neemann/digital/draw/model/Net.java
+++ b/src/main/java/de/neemann/digital/draw/model/Net.java
@@ -27,6 +27,7 @@ import java.util.HashSet;
  * After creation all the ObservableValues belonging to the outputs are set.
  */
 public class Net {
+    private static final ObservableValue UNCONNECTED_WIRE = new ObservableValue("unconnected wire", 1).setToHighZ().setConstant();
 
     private final HashSet<Vector> points;
     private final ArrayList<Pin> pins;
@@ -172,14 +173,17 @@ public class Net {
                 outputs.add(p);
         }
 
-        if (outputs.size() == 0)
+        if (outputs.size() == 0 && inputs.size() > 0)
             throw new PinException(Lang.get("err_noOutConnectedToWire", this.toString()), this);
 
-        ObservableValue value = null;
+        ObservableValue value;
         if (outputs.size() == 1 && outputs.get(0).getPullResistor() == PinDescription.PullResistor.none) {
             value = outputs.get(0).getValue();
         } else {
-            value = new DataBus(this, m, outputs).getReadableOutput();
+            if (inputs.size() == 0 && outputs.size() == 0) // unconnected wire
+                value = UNCONNECTED_WIRE;
+            else
+                value = new DataBus(this, m, outputs).getReadableOutput();
         }
 
         if (value == null)